I have no experience in this area, but I bought Peggie's book. It's on the boat though, so I cannot reference it for this post.

Do you have the room to mount the second tank nearby and at or below the same level as the original? If so, an overflow line from tank 1 into the new tank 2 plus the required vent and pumpout lines. An alternative would be to plumb a selector valve into the outflow line from the head to select the tank. One line goes to tank 1, the other to tank 2. Each have their own vent and pumpout lines. The second method is more flexible for tank placement and does not require you to modify the existing tank with an overflow line to tank 2.

I suspect selector valves in the outflow lines might be culprits for flow restrictions if someone flushes something they shouldn't. At least they'll be accessible and you'd know where to look first.

An alternative would be to plumb a selector valve into the outflow line from the head to select the tank. One line goes to tank 1, the other to tank 2. Each have their own vent and pumpout lines. The second method is more flexible for tank placement and does not require you to modify the existing tank with an overflow line to tank 2.

Doesn't that just treat the waste so it can be pumped overboard legally in some areas? As I understand it, there are many places on this mini loop that are no discharge, treated or not, and where even your ability to pump overboard must be locked out or disconnected altogether.

To answer the first question, my tank capacity is 14 gallons. That is pretty minimal and I've considered a treatment system or at least a low water use head. I'll often just "go" off the swim platform if nobody is around and we're not underway. For some reason, it's legal to pee (or poop) directly into the water, but not if you've put it in something first.

For the temporary extra capacity, instead of modifications to the boat, how about taking a porta potty along on the trip?

My holding tank capacity is 40 gallons. When used on a long trip by 2 men, it would fill up in about 4 days with the old head. Now that we've installed the Jabsco Quiet Flush, it lasts more than a week, but I haven't yet found out how much more.

Flywright;
Do you flush with salt water or fresh? My Californian's tank capacity is only 30 gallons; lasts 2 of us about 3 -4 days.I'm sure we have some scale build up that is reducing capacity. But I'm not sure that we can afford to give up fresh water supply for flushing. How does your setup work? Thanks

For what it is worth, my 200-gallon fresh water, 35-gallon holding tank, system seems reasonably balanced although I haven't tested its limits. Like to think a fresh-water toilet system is easier on the components.